1. Field of the Invention
The present invention relates to a process for producing a chromium-containing steel sheet hot-dip plated with aluminum, improved both in aluminum hot-dip plating characteristics and in plating adhesion.
2. Description of the Related Art
Ferrous sheet materials having superior resistance to corrosion, heat and oxidation have been known, a typical example being aluminum-plated carbon steel sheets and stainless steel sheets. Among these known materials, aluminum-plated carbon steel sheet is less expensive as compared with a stainless steel sheet containing about 7 wt. % of chromium and yet exhibits superior resistance to corrosion, heat and oxidation which well compare with those exhibited by the above-mentioned stainless steel sheet. For these reasons, the aluminum-plated carbon steel sheets are widely used in structures which are required to have high resistance to corrosion, heat and oxidation, such as exhaust pipes of automotive engines, for example.
In recent years, however, the use of aluminum-plated carbon steel sheets is being restricted for prevention of environmental pollution. On the other hand, the current trend for higher performance of automobiles has given rise to the demand for materials which have higher resistance to corrosion, heat and oxidation.
In particular, corrosion resistance of this type of material is significantly influenced by the quality of the plating, since any flaw in the plating layer allows generation of rust on the surface of the underlying metal exposed through the flaw. Such rust tends to rapidly grow to cause pitting to perforate or corrode the material quite rapidly.
There is also a trend toward automobiles of front-engine front-drive type in which the distance between the engine and the exhaust muffler is rather long. In addition, automobiles in city areas are usually driven only short distances so that engines are often turned off before the temperature in the muffler is raised to a high temperature. Consequently, exhaust gases are cooled and condensed into liquid form in the exhaust pipes and mufflers to promote corrosion of the exhaust pipes and mufflers by corrosive ions such as SO.sub.4 --, Cl-- and CO.sub.3 -- which are contained in the condensate stagnating in the exhaust pipe and muffler. Higher corrosion resistance is required also from this point of view.
Under these circumstances, 11%-chromium stainless steel and 13% chromium stainless steel, which can be produced at low cost and which exhibit superior corrosion resistance, are becoming popular as the materials of exhaust gas-systems of automotive engines which are required to have high corrosion resistance, particularly in North American countries. These materials, however, are still unsatisfactory in that red rust tends to be generated after the material is formed into an exhaust pipe, particularly at weld portions of the pipe.
In order to obviate thee problems, U.S. Pat. No. 4,675,214 proposes the use of an aluminum hot-dip plated stainless steel in which stainless steel as a corrosion-resistant base material is hot-dip plated with aluminum.
This material exhibits superior resistance to corrosion at portions of underlying base material exposed through flaws in the plating layer, as well as at welded portions, thus providing an effective countermeasure to pitching corrosion which has been one of the critical problems.
The aluminum hot-dip plated stainless steel disclosed in the above-mentioned United States Patent is produced by a process in which a stainless steel sheet is dipped in a plating bath of molten aluminum, after the surface of the steel sheet is cleaned by treatment of the steel in a reducing gas atmosphere for reducing oxides of chromium, silicon and manganese which are densely generated on the pole surface of the steel sheet.
The reduction of these oxides essentially requires that the reducing gas atmosphere be controlled to contain hydrogen gas of high density and oxygen gas of low density and low dew point. Formation of such a reducing gas atmosphere requires expensive and complicated equipment, as well as complicated control.
It is true that any oxide mentioned above, remaining on the surface of the stainless steel sheet to be plated, is reduced during subsequent dipping due to the strong reducing effect produced by the aluminum so that the surface of the steel sheet is cleaned. However, if nitrogen is contained in the reducing gas, CrN is generated on the surface of the steel sheet so as to impair generation of aluminum-iron alloy layer on the steel sheet surface during dipping in the aluminum bath, resulting in a plating defects.
Thus, it has been necessary to reduce the nitrogen gas content while increasing the hydrogen gas content in the atmosphere gas.
The aluminum-iron alloy layer which is formed on the steel sheet surface during aluminum hot-dip plating is generally fragile. Therefore, this alloy layer which forms the boundary between the plating layer and the iron as the base metal when, for example, the plated sheet is subjected to bending work, particularly when the thickness of this alloy layer is large, thus increasing the risk of separation of the aluminum plating layer.